Sealed lubricated bearings in rotary rock bits are well known in the oilfield. Herein a lubrication reservoir having pressure differential compensation means is generally remotely located from the bearing structure. A lubrication passage extends from this reservoir to the particular cone bearing it serves. The lubricants are generally of a type having a high temperature resistance, such that they are not degraded due to the operating conditions encountered in the well borehole. Characteristic of all these systems is that they are enclosed and covered for protection from the drilling environment. This necessarily means there is no readily available way to indicate the quantity of lubricant in a particular lubrication reservoir at the time the rock bit is pulled form the hole or attached to rerun.